1. Field of the Invention
This invention relates to processes for producing the drawn molded article of a fluoroplastic (a fluorine-contained resin), more particularly to a process for producing a drawn molded article of a fluoroplastic which is significantly improved in mechanical properties such as strength, tensile modulus and the like by drawing a fluoroplastic under the specified conditions.
2. Prior Art
Fluoroplastics have been widely used as engineering plastics because their remarkably high melting point, excellent resistance to chemicals and low friction coefficient of the surfaces. Since a fluoroplastic resembles in chemical structure closely a polyolefin, it has been attempted to produce a drawn article in the form of a film, tape and yarn of high strength and tensile modulus by stretching the molecular chains having been changed to be uniaxially oriented and highly crystallized morphology. Japanese Laid-Open Patent Publication Nos. 1-192812, 2-307907 and 5-78908 disclose the process of producing a fluoroplastic fiber by applying a technique of drawing polyethylene or polypropylene. One of these prior techniques involves a complicated processes in which after dissolving a fluoroplastic which is not dissolved with a general-purpose solvent, with a special solvent thereby obtaining the solution, the fluoroplastic is drawn by spinning, followed by extracting the solvent. In another technique, a fluoroplastic solution is melted by heating at a temperature higher than the melting point thereof and the resulting high viscosity melt is drawn by spinning. However, it is very difficult in technical view to spin the solution and the melt uniformly and since particularly the latter method is effected at a high temperature above the melting point, there are anxieties of the occurrence of gases which thermally decomposes the fluoroplastic and is harmful for a human body. Therefore, the fact is that the above-mentioned two methods are inadequate for practical use. Other than these prior methods, there are another methods of extruding polytetrafluoroethylene in the solid state as disclosed in J. Polym. Sci. Polym. Phys. Ed., 17 (1979) 73 and Polym. Eng. Sci., 26 (1986) 239. However, these methods fail to improve the strength of the drawn article to an extent that would be satisfactory.
Furthermore, although it is disclosed in Japanese Laid-Open Patent Publication No. 8-132521 that a drawn article is obtained by tensile stretching an article derived from solid-state extrusion, this method has been demanded to be further improved.
Generally speaking, a drawn article can be improved in strength and tensile modulus with respect to the drawing direction by using a starting fluoroplastic having a larger molecular weight if starting fluoroplastics are intended to be drawn at the same draw ratio or by increasing the draw ratio if starting fluoroplastics are the same in molecular weight. However, in the already mentioned methods the use of a starting fluoroplastic excessively increased in molecular weight leads to a reduction in solubility in a solvent or an increase in viscosity when melted, resulting in not only an inconvenience that spinning or drawing operation is hindered but also a risk that the breakage caused by drawing occurs frequently.
It has been found that there can be produced a drawn molded article of a fluoroplastic excelled in mechanical properties such as strength and tensile modulus by drawing in a specified manner.